1. Field of the Invention
The invention relates to an electron tube control grid. The field of the invention is that of electron tubes, especially grid-based electron tubes working with longitudinal electron beams such as, for example, inductive output tubes. Hereinafter in the description, this type of tube shall be referred to as an “IOT”. It is understood that the invention is not limited to IOTs.
An IOT comprises an electron gun that emits an electron beam, a resonant cavity crossed by the beam and a collector that collects the electrons from the beam at their exit from the cavity.
For a clearer understanding of the invention, an IOT is shown in FIG. 1 which is a view in longitudinal section of an exemplary electron tube represented partially and comprising a cathode 1 and a control grid 2, both shaped for example as spherical segments. The cathode 1 and the control grid 2 form the electron gun. The cathode 1 has an emitting part 3 in the shape of a spherical segment that demarcates a substantially central hollow portion 4. An anode 5 and a heating device 6 of the cathode 1 are also shown in this figure. The electrons emitted by the cathode 1 are recovered at the end of travel in a collector 7. The concave face of the cathode 1 emits electrons when it is taken to high temperature by means of the heating device 6. The electrons go through the control grid 2 and penetrate a resonant cavity 8, forming a longitudinal beam before they reach the collector 7.
The control grid 2 is used to modulate the emission of electrons so as to obtain a variation in the density of the beam before it enters the resonant cavity 8. The control grid has a solid central hub 9 around which bars 10 have been developed. The electron beam passes between these bars. The bars are fixed firstly to the central hub 9 and secondly to the body of the electron tube by means of an outer collar 11 belonging to the grid 2.
2. Description of the Prior Art
FIG. 2 gives a view, in a plane perpendicular to the axis of the electron beam, of a prior art plane control grid 15. This grid has several concentric circular bars 16 held together by means of radial bars 17. The bars 17 are fixed to the outer collar 11 and, in the case of some of them, to the central hub 9. The concentric circular bars 16 are arranged at a constant interval or spacing on the surface crossed by the electrons. On this constant nature of the interval depends the quality of the electron beam in terms of homogeneity, density of flow and speed of the particles. This constant nature also affects the transconductance of the electron tube. The transconductance is defined by Δi/Δv where Δi represents a variation of intensity at the anode 5 and Δv represents a variation of voltage between the control grid 2 and the cathode 1.
FIG. 3 is a view in volume of a prior art spherical control grid 20. The working of the control grid 20 is similar to that of the control grid 15 shown in FIG. 2. The control grid 20 has bars 21 laid out at a constant interval on the surface crossed by the electrons. The bars 21 represent parallels or lines of latitude of the spherical surface formed by the control grid 20. The control grid 20 also has bars 22 whose function is equivalent to that of the bars 17 shown in FIG. 2. The bars 22 are fixed to the outer collar 11 and some of them are fixed to the central hub 9. The bars 22 represent meridian or longitude lines of the spherical surface formed by the control grid 20. The term “meridian lines” and “parallels” are defined by analogy with the terrestrial sphere.
In these two prior art control grids 15 and 20, the bars 16 and 21 control the electron beam. More specifically, the flow rate of the electrons coming from the cathode is controlled by the potential of the control grid used. Depending on its potential, the grid pushes back the electrons to the cathode 1 or lets them pass through toward the anode 5. On the contrary, the radial bars 17 or meridian lines 22 have a rather disturbing effect on the control of the electron beam. They intercept a part of the electron beam. The utility of the bars 17 and 22 is limited essentially to the mechanical holding of the bars, 16 and 21 respectively, as well as to the conductive discharge of the heat generated in the control grid by the interception of a part of the electron beam and by the radiation of the cathode 1. The heat is discharged to a major extent toward the outer collar 11. It is important to note that the part of the heat generated in the control grid is due to the interception of a part of the electron beam by the bars 17 and 22.
The invention proposes a solution to this problem by limiting the presence of bars that make little contribution to the control of the electron beam. This is done without impairing the heat conduction of the control grid.